If its what I think it is -- you probably corrupted Maya by importing so many damn polygons. Terragen has an issue (or is that TerrainGen? Get the two mixed up) of occasionally creating like 12x the normal polycount necessary (duplicate faces, faces with 0,0,0 as a vertex, etc.).

Try to create a "heightmap" on Terragen and use MhCannon's "bake displacement" tutorial on This thread 7th post down. It'll basically warp your surface into the topographical view of your map (which is what you want).

The last time I tried to import an OBJ file from one of those two programs, I think its Terragen, I ended up with my viewport just an explosion of lines and a BSOD (well, WinXP equivalent "Error writing (blah blah) closing".